This research program is directed at elucidating the mechanism of action of steroid hormones in bone cells. Bone is a clinically important target organ for glucocorticoids, Vitamin D metabolites and possibly for other classes of steroid hormones including mineralocorticoids, androgens and progestins. The goals of this proposal are to determine which receptors are present in bone and to characterize and to define the actions of the various steroids on bone cells. The long range objective is to learn how glucocorticoids, Vitamin D metabolites and other steroid hormones effect bone cell function and thereby gain insight into both normal bone physiology as well as the pathophysiology of metabolic bone diseases such as osteoporosis and osteomalacia. The system to be employed is a primary monolayer culture of bone cells obtaind from fetal rat or newborn mouse calvaria by collagenase digestion. Attempts will be made to resolve this heterogeneous cell population into subpopulations enriched for osteoblasts and osteoclasts. The focus of attention in the forthcoming grant period will be to expand on our previous characterization of glucocorticoid receptors as well as to fully define the 1,25(OH)2 Vitamin D receptor which we have recently described. Bone cells will be scrutinized for the presence of receptors for the other classes of steroid hormones as well. A second major effort will be directed at functional responses to the steroid hormones. Correlations will be made between receptor binding studies and functional responses in an attempt to link biological expression of the hormone to the initiating event at the receptor site. Functional studies will include analysis of the cyclic AMP generating system, collagen synthesis and enzyme activity. In both the receptor and the functional work, a major theme to be emphasized is the interaction between hormones including the bone-active peptides parathyroid hormone and calcitonin and the various steroid hormones.